As one of devices for converting NOx contained in a diesel exhaust gas into harmless substances, urea selective catalytic reduction (hereafter, abbreviated as SCR) has been brought into practical use.
The SCR allows a urea solution injected through the inlet side of an SCR reactor to undergo hydrolysis with heat provided by an exhaust gas and thereby produce ammonia. Then, with an SCR catalyst in the SCR reactor, the nitrogen oxide (NOx) in the exhaust gas and ammonia are subjected to the reduction reaction to be converted to nitrogen and water and thereby made harmless.
Furthermore, a diesel particulate filter (DPF) for capturing a diesel particulate matter (hereafter referred to as the PM) and a diesel oxidation catalyst (DOC) for oxidizing unburned fuel are connected to the upstream of the SCR. The temperature of an exhaust gas passing through the SCR reactor varies in a wide range from 200° C. at the time of low load (for example, at the time of starting the engine) to 600° C. at the time of DPF regeneration.
Here, in recent years, it has been studied to apply a catalyst carrier having a honeycomb structure which carries a catalyst to the SCR, DOC, and the like. Such a catalyst carrier for purifying an exhaust gas is known (for example, see Patent Literature 1) in which a roll-shaped honeycomb structure wound around in layers with strip-shaped metal corrugated foil and flat foil is inserted into an outer jacket, and a catalyst substance is caused to adhere to the corrugated foil and the flat foil. In the catalyst carrier, the corrugated foil is provided with a number of fins having different phases between adjacent ones in the axial direction of winding (i.e., an offset structure).